Thermal switch



April 1951 M. K. PARKHURST 2,548,050

THERMAL SWITCH Filed Sept. 28, 1949 //7 van for Ma/co/m K Parkhursf Attorneys Patented Apr. 10, 1951 UNITED STATES PATENT OFFICE THERMAL SWITCH Malcolm K. Parkhurst, New York, N. Y., assignor to Control Products Inc., Harrison, N. J a corporation of New Jersey Application September 28, 1949, Serial No. 118,439

3 Claims.

This invention relates to thermal switches for electrical circuits, more particularly to thermal switches of small dimensions for use in high temperatures and especially adapted for use in fire-detection circuits.

The principal object of the invention is to provide such a switch which has no wearing surfaces which might throw the switch out .of adjustment when it is subjected to long periods of sustained vibration as, for example, when used in the fire-detecting systems of airplanes.

Another object is to provide such a switch which is capable of being calibrated and thereafter hermetically sealed.

Another object is to provide a switch having terminal means subject to possible distortion when the switch is being connected in the circuit in which it is used, said terminal means being independent of the parts of the mechanism by which calibration is efiected so that such stresses, or accidental stresses or strains, will not disturb the calibration.

Another object is to provide a switch having no spring contacts having initial bends creating stresses therein which may relieve themselves, thus destroying the calibration.

Other objects and advantages will in part appear, and part will be obvious from the following detailed description of the present preferred embodiment of the invention, herein .described for purposes of illustration, taken in connection with the drawings, in which,

Fig. 1 is a central sectional elevation through a complete switch which has been calibrated and sealed;

Fig. 2 is a plan View of the switch;

Fig. 3 is an elevation of the movable contactcarrying head attached to the rod taken on the line 33 of Fig. 1, and associated parts of the switch which are shown in central section, and

Fig. 4 is a plan view of the fixed contact member.-

In describing the embodiment shown in the drawings, the construction of the completed switch will first be described followed by explanations of its mode of assembly and of its operation. Structure The switch embodies a rod and tube type thermal element in which the rod and tube have .difierent thermal .coefiicients of expansion, the

tube being attached to a housing carrying a fixed electrically connected with another portion of 2 the housing serving as a terminal, the end of the rod being provided with a head which is slidable within the housing and carries the adjustable moving contact. As shown, the switch is designed to be normally open and to close upon the attainment of a predetermined elevated temperature.

Referring to the drawings, the switch comprises a tube I G of suitable alloy such as Inconel and a rod ll of a material having a lower .coetficient of expansion, such as 42% nickel alloy. The specific materials of the tube and rod are, of course, subject to selection in accordance with the temperature ranges in which the switch is designed to operate, the selection being made in accordance with well understood principles. The lower or outer ends .of the rod and tube are permanently and rigidly joined together in any suitable manner, preferably by means of the cupshaped plug I2 which is spot welded to the tube and to the center of which the rod H is riveted at 14. Preferably, the end of the unit is her metically sealed by silver solder applied over the riveted end of the rod as shown at 5 and around the edge of the plug as shown at I 6.

At its upper or inner end, the tube [0 is slipped over a cylindrical tubular boss I8, which forms part of the housing body l9 and is fastened thereon by spot welding. A mounting flange 21), which is provided with a central aperture to accommodate tube IO. underlies body i9 and is secure-d thereto by means of spot welding. Silver solder 2| hermetically seals the joint between the tube l0 and boss [8.

Body It provides a cylindrical chamber defined by its upstanding side wall which, prior to assembly, is not flanged over, as shown at 22, :but stands straight up. Within this chamber is a mica washer 24 which rests on the bottom of the chamber and has an outer diameter corresponding to the diameter of the chamber. Resting on this washer is the fixed contact member shown in Fig. 4 which has a circular body 25 and atongue portion 25, preferably provided with a separate contact element 28. This contact member is flat and the tongue 26 is resilient but sufficiently rigid to maintain its position under vibration. Being fiat, this contact element contains no stresses, such as would be present if the contact were bent, so that calibration is main.- tained.

The second part of the housing is in the form of a hollow metal cap 29 in electrical contact with the fixed contact member through its outwardly extending flange .39, the outer diameter of which is somewhat less than the inner diameter 3 of the chamber in the body I 9, so that it is spaced therefrom. Over flange 301s a second mica washer 3i and a rigid metal washer 32 over which the flange 22 is turned to rigidly hold these parts together. The upper end of cap 29 is initially open and is counter-bored to provide a shoulder 34 surrounded by an initially cylindrical wall 35. Seated on the shoulder 34 is the annular flange 36 of a terminal plug 38 which is held in place by flanging over the wall 35, as shown at 39. The joint is then soldered at 49 to form an air tight seal. Plug 38 is internally bored and tapped, leaving its bottom closed, to receive the screw 4| under the head of which there is a washer 42. Instead of a female screw socket, the terminal plug may be provided with a threaded post.

At its inner or upper end the rod I I is provided with a head 44 having a lower sleeve 45 closely fitting the rod and an upper internally threaded sleeve 46 which is preferably longitudinally split, as shown at 48 in Fig. 3, in which is inserted the contact-carrying screw 49 provided at its lower end with a suitable contact element 59. The sleeve 45 is fixed on rod II by welding; It is to be understood that the head may be integral with the rod, if desired. Sleeve 46 is split to provide tension on the screw 49 to hold it in its adjusted position. Screw 49 is located on a prolongation of the longitudinal axis of the rod II and its lower end, which carries the contact, extends into an aperture 5I which extends through the head 44 in a crosswise direction, at a right angle to the rod axis. The body of head 44 containing this aperture is so dimensioned as to have a sliding fit in the bore of boss I8. When finally assembled and adjusted, the contact elements 28 and 59 are spaced apart so as to provide a nor mally open switch.

The space existing between cap 29, the inner edges of washers 3| and 32 and of flange 22 of body I9 is filled with a fused glass seal 52 which may extend over the flange 22.

Assembly The structure just described is assembled in the following manner: a sub-assembly is formed comprising the head 44 which is Welded to one end of rod II and the plug I2 which is riveted to the other end of the rod. The second subassembly is formed by welding tube II) to the boss I8 and by welding the mounting plate 29 to the underside of body I9, the common joint being filled with solder 2|. The first sub-assembly is then inserted in the tube of the second subassembly, leaving the aperture 5| in the head 44 in a position above the top of the side wall of the body I9. The mica washer 24 is then dropped into the body, the tongue 26 of the fixed contact is inserted in the aperture 5| and the first sub-assembly is lowered into the body to its desired final location, which may be determined by means of a gauge designed to approximately center tongue 25 in the aperture 5I. The two sub-assemblies are then connected by welding the plug I2 to the tube Ill and coating the joints with the solder shown at I5 and I6. The contact body 25 is centered in the housing body I9 by means of a jig and the contact may be temporarily held in place by a spot of adhesive. Cap 29 is then positioned over contact body 25, washers 3| and'32 are put in place and, while the cap is held in centered position, these parts are locked in body I 9 by forming the flange 22 under very high pressure in a press. The glass seal 52 is then applied in the form of a slurry which is baked and fired. The contact-carrying screw 49 is then inserted in the split neck 46 and adjusted to contact-closing position while the rod and tube are subjected to a fixed elevated temperature, the screw being adjusted to the position where the contact elements 28 and 50 are just brought into engagement. The position of screw 49 is then fixed by means of a seal 54 of chemically-setting heat-resisting cement. The terminal plug 38 is then placed in the open end of cap 29 and the flange 39 is formed and the joint is soldered at 40. The insertion of screw 4I completes the assembly.

As will be seen, the movable contact element 50 is connected to ground through the mounting flange 29 and fixed contact 26 is connected to the live circuit through cap 29 and the terminal plug 38 and screw 4I.

, Operation When the tube I9 is subjected to high ambient temperature it expands, carrying with it rod II which has a lower coefiicient of expansion. Head 44 moves with the inner end of the rod, sliding in boss I8 and moves the contact 50 toward contact 28. These contacts will close when the temperature is reached for which the switch is callbrated.

It will be observed that in the structure above described there are no points of possible Wear which can affect the calibration of the switch, as in other structures Where adjustment between two contacts is obtained, for example, by an adjusting screw the end of which bears against a contact or its carrier and in which the two surfaces may have relative movement when the device is subjected to vibration. The only relatively moving parts in the finally assembled device are the head 44 and the boss I8 but any wear taking place between these parts has no effect on the adjusted spacing of the contacts.

Furthermore, when a wire is connected to the device by means of screw 4|, cap 29 may be subjected to stress. Even if this cap is moved or bent or distorted, it has no effect on the adjustment of the contacts because their spacing is independent of the position of the cap.

The final closure of the assembled switch by the terminal plug 38 permanently seals the switch against the possibility of tampering with the adjustment of screw 49.

While the invention has been described and illustrated by reference to one embodiment thereof, it is to be understood that it is not limited thereto since various details of construction may be changed without departing from the spirit of the invention. For example, the specific form of the aperture 5| in head 44 is capable of many modifications and the tube I0 might be secured to the inner surface of boss I8 instead of toits outer surface. Other possible modifications will readily be apparent to those skilled in the art and the invention is therefore to be construed broadly within the purview of the claims.

What is claimed is:

1. In a thermal switch, for use as a fire detector or the like, of the type having a rod and concentric tube of different thermal coeflicients of expansion connected at their outer ends and having a two-part housing attached to the inner end of the tube, said housing comprising a body and a cap; a fixed electrical contact member rigidly supported in said body and insulated therefrom and having an integral resilient tongue portion extending over the end of said rod, the inner end of said rod having a head provided with an aperture into which said tongue extends, said head having a threaded sleeve, a threaded electrical contact adjustably positioned in said sleeve and adapted to engage said tongue, said head being slidably supported in said housing, said cap being in electrical engagement with said fixed contact and having a bore in alignment with said sleeve through which said threaded electrical contact can initially be adjusted, and a terminal plug forming a closure for said bore and hermetically sealed therein.

2. A normally open thermal switch adapted to close when subjected to excessive temperatures and capable of withstanding long periods of vibration in its normally open position without losing its calibration due to the absence of any wearing points therein affecting calibration, said switch comprising a rod and concentric tube of difierent thermal coefiicients of expansion connected at their outer ends, a housing attached to the inner end of the tube and having a bore adjacent the tube the axis of which bore is aligned with the axis of the rod, a head carried by said rod slidably supported in and bearing against the wall of said bore, said head having a transverse opening communicating with a threaded opening which is on the opposite side of the head from said rod and aligned therewith, a fixed electrical contact member rigidly supported in said housing and having a stiff but resilient integral tongue portion extending into said transverse opening, an adjustable contact screw in said threaded opening adapted to make contact with said tongue portion at excessive temperatures, said housing having a passage through which said contact screw may be adjusted, and means for closing said passage after such adlustment.

3. A normally open thermal switch adapted to close when subjected to excessive temperatures and capable of withstanding long periods of vibration in its normally open position without losing its calibration due to the absence of any wearing points therein afiecting calibration, said switch comprising a rod and concentric tube of difierent thermal coefiicients of expansion connected at their outer ends, a housing attached to the inner end of the tube and having a bore adjacent the tube the axis of which bore is aligned with the axis of the rod, a head carried by said rod slidably supported in and bearing against the wall of said bore, said head having a transverse opening communicating with a threaded opening which is on the opposite side of the head from said rod and aligned therewith, a fixed electrical contact member rigidly supported in said housing and having a stifi but resilient integral tongue portion devoid of bends creating residual stresses therein and extending into said transverse opening, an adjustable contact screw in said threaded opening adapted to make'contact with said tongue portion at excessive temperatures, said housing having a passage through which said contact screw may be adjusted, and means for closing said passage after such adjustment.

MALCOLM K. PARICHURST.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,496,229 Parkhurst Jan. 31, 1950 FOREIGN PATENTS Number Country Date 254,821 Great Britain July 15, 1926 268,645 Great Britain Apr. 7, 1927 334,359 Italy Jan. 24, 1936 779,904 France Jan. 24, 1935 

